UC San Diego

It is widely accepted that mesopelagic fishes are very abundant and maintain a high global biomass, but there exists a high degree of uncertainty in these biomass estimates. These fishes carry out all of their life stages in the water column and many undergo daily vertical migrations from the depths of the mesopelagic to the surface waters at night, returning to depth at dawn. This migratory behavior is one of the ecological factors that complicates our abilities to confidently evaluate the biomass of this group of fishes. Assessing the habitat use throughout ontogeny of mesopelagic fishes is a critical first step in understanding their role in the ecosystem. Additionally, the onset of diel vertical migration is relatively known for these fishes. Furthermore, larval fish identification is difficult and time consuming, and is often resolved only to the family level due to lack of taxonomic knowledge of species-specific ELH stages. Larval fishes

undergo profound changes during the early life history stages, therefore it is plausible that the earliest stages respond to environmental perturbations very differently than the later stages.

This dissertation is an examination of the importance of ontogenetic stage-specific investigation of larvae relative to habitat use. I describe the vertical distribution of ELH stages of the common mesopelagic fish species off central California by analysis of repeated tows of a discrete depth sampler, collected with a 1m2 MOCNESS from the same station during both day and night. I determined that common species of mesopelagic fish larvae off central and southern California exhibit differences in their diel distributions and that there are substantially more species of mesopelagic fish larvae below the epipelagic zone. Investigation using a larger sample set of MOCNESS data collected in central and southern California confirmed that mesopelagic fish larvae are more deeply distributed than previously realized. The patterns suggested that DVM begins in the larval stages for some species, which is much earlier in development than previously described for species in this geographic region. I then describe the ontogenetic changes in abundance and horizontal distribution of common species of mesopelagic fish larvae affected by the extreme El Nino event in 1997-1998 followed by the La Nina in the California Cooperative Oceanic Fisheries Investigations (CalCOFI). The results indicated that within the CalCOFI sampling area, mesopelagic fish species with an affinity for warm water conditions had a higher larval abundance and were closer to shore during the El Niño, and were less abundant and farther offshore during the La Niña. The opposite pattern was generally observed for mesopelagic fishes with an affinity for cold water conditions. Additionally, I discovered that the mesopelagic fish larval abundance in the CalCOFI region is dominated by the earliest stages of the larval period.

Finally, I discuss the conclusions of this dissertation that highlight the importance of ontogenetic habitat use patterns of mesopelagic fishes. Given their high global abundance, importance in the oceanic food web, and the potential for fisheries exploitation, this is an essential first step towards a reliable biomass assessments of mesopelagic fishes.